Increasing use of high frequencies in radio frequency systems has led to a need to modify and adapt existing antenna structures. Driving antennas at a higher frequency tends to affect directivity and thus affecting the effective range of antennas. As discussed in Christopher Coleman's Basic Concepts, An Introduction to Radio Frequency Engineering, Cambridge University Press (2004), in EM, directivity is a property of the radiation pattern produced by an antenna. Directivity is defined as the ratio of the power radiated in a given direction to the average of the power radiated in all directions; the gain pattern is the product of the efficiency of the antenna and the directivity.
For example, FIG. 1 shows an antenna, frequently called a discone antenna, composed of a disc 1, a frustum circular conic section structure 3, conductors 7 and a voltage source 9 with a throat or feed gap 5, typically connected in such a manner as to have an axis of rotational symmetry 15. FIG. 2A shows the FIG. 1 antenna with an axis of rotational symmetry 15 that is perpendicular to the disc 1 and runs through the center of the cone structure 3. Discone antennas provide azimuthally (defined as the plane orthogonal to the axis of symmetry of the antenna and parallel to the disc component of the antenna) omni-directional field (radiation intensity) patterns over broad frequency ranges.
FIG. 2B shows an exemplary omni-directional radiation pattern. In particular, FIG. 2B shows an antenna with an elevation pattern 13A that is substantially directed perpendicular to the axis of symmetry 15, having a direction of the peak magnitude 11 of the elevation pattern.
FIG. 2C shows an exemplary radiation pattern at a higher frequency where the resulting elevation pattern 13B is oriented away from the axis perpendicular to the axis of symmetry by an angle 17 greater than 90 degrees. The FIG. 2C radiation pattern shows a maximum radiation intensity oriented toward the cone portion of the antenna. The direction from the origin of the spherical frame of reference for the antenna through the peak of the intensity pattern is defined by a function here represented by the direction of the pattern peak vector 11 when the elevation pattern is not parallel with the plane of the disc component of the antenna. The included angle 19 defines the degree of flair for the cone from the lower portion of the axis of symmetry 15. If a discone antenna with the radiation pattern as represented in FIG. 2C were mounted on a vehicle, for example, the direction of pattern peak would increasingly be below the horizon as frequency was increased, thus reducing the range and effectiveness of such a discone antenna.
Accordingly, there is a need for an improved antenna design which provides improved directional gain that also has a simple and highly durable design.